Clean up git tree

This removes the ununsed `obj` directory. It also suggests in the README
to create the "coverage" files in a separate directory and adds the
coverage files to .gitignore.

readme: Improve instructions for coverage reports

README.md

@@ -96,7 +96,8 @@ To create a report, `gcovr` is recommended, as it includes branch coverage repor
 
 To create a HTML report with coloured and annotated source code:
 
-    $ gcovr --exclude 'src/bench*' --html --html-details -o coverage.html
+    $ mkdir -p coverage
+    $ gcovr --exclude 'src/bench*' --html --html-details -o coverage/coverage.html
 
 Reporting a vulnerability
 ------------